In general, a photographic material is produced by coating at least one photographic layer on a plastic film support. The plastic film support generally employed is a cellulosic polymer such as triacetyl cellulose (hereinafter referred to as "TAC") or a polyester polymer such as polyethylene terephthalate (hereinafter referred to as "PET").
A photographic material is generally grouped into two forms: one being a sheet film such as an X-ray film, a film for photomechanical processes and a cut film, and the other being a roll film such as a color or black-and-white negative film having a width of 35 m/m or less. The latter is generally housed in a patrone (cartridge) and is charged in a camera for picture-taking.
As a support for a roll film, TAC is commonly used. The most characteristic features of TAC is that it has no optical anisotropy and has a high transparency. Another excellent characteristic is that TAC has an excellent property of easily smoothing the curl of a developed photographic material in which it is used as a support. Specifically, since a TAC film has a relatively high water-absorbing property (though being a plastic film) because of its characteristic molecular structure, the molecular chain of the film becomes fluid after the TAC support film has absorbed water during development of a curled roll film so that the molecular chain fixed in the curled roll film is rearranged.
As a result, TAC displays an excellent property of easily smoothing a curled roll film in which it comprises the support. However, a photographic material having a support without such an easily curl-smoothing property involves various problems, for example, scratching and out-of-focusing during the printing step of forming an image on a photographic printing paper from the developed roll film of the material and also jamming during feeding of the developed roll film of the material in the printing step.
On the other hand, since a PET film has excellent producibility, mechanical strength and dimensional stability, it has heretofore been considered to be substitutable for TAC. However, when the PET film is used as a support of a photographic material of the most popular roll film, it strongly curls and the curl of the film strongly remains even after development. As a result, the handling of the curled film is difficult and troublesome after development. Therefore, despite the above-mentioned excellent properties, the use of PET in the field of photographic materials has heretofore been limited.
For reducing the curling property of a polyester film, various attempts have heretofore been made. For instance, a method is described in JP-A-51-16358 (U.S. Pat. No. 4,141,735) (the term "JP-A" as used herein means an "unexamined published Japanese patent application") in which a polyester film is heat-treated at a temperature lower than its glass transition temperature by 5.degree. C. to 30.degree. C.
The use of photographic materials is currently diversified widely, and the technology for rapid feeding photographic film in a camera or the like during picture-taking, elevation of image magnification, and reduction of the size of picture-taking devices has advanced noticeably. With this advanced technology, the support of photographic materials must have high strength and high dimensional stability and be as thin as possible.
In addition, with the great reduction of the size of picture-taking devices, the demand for reducing the size of film-housing cartridges has also become great. Heretofore, a 25 mm-diameter cartridge has been used in a 135 system picture-taking devices. In the case, therefore, where the size of the spool is reduced to 10 mm or less and additionally the thickness of the TAC support of the roll film to be charged in the current 135 system device is reduced from 122 .mu.m to 90 .mu.m, the size of the cartridge can be reduced to have a diameter of 20 mm or less. On the other hand, if the diameter of the spool is reduced to 3 mm or less, the roll film in the cartridge unfavorably suffers from pressure marks. Therefore, further reduction of the size of the spool below the described limit would be impossible.
Reduction of the size of a roll film cartridge involves two problems. The first problem is that the reduction of the thickness of the roll film to be in the cartridge is often accompanied by lowering of the mechanical strength of the film itself. In particular, the bending elasticity of a roll film decreases in proportion to the third power of the thickness thereof. A silver halide photographic material generally comprises a light-sensitive layer of a gelatin dispersion on a support, and the layer is shrunk under low humidity conditions to curl to a gutter-like form. The support is therefore needed to have a bending elasticity to be resistant to shrinking stress.
The second problem is that the roll film in a small-sized cartridge with a small-sized spool is strongly curled during storage of the film. In the conventional 135 system device, the smallest diameter of a roll film of 36 exposures housed in the cartridge is 14 mm. If the cartridge is desired to be smaller so that the diameter is 10 mm or less, the hardly smoothable curl of the roll film housed in the cartridge would be much greater causing various troubles in handling the exposed film. For instance, if the exposed film taken out of such a small-sized cartridge is developed in a mini-laboratory automatic developing machine, the film would be curled up during handling since only one edge of the film is fixed to the leader but the other one is not, so that feeding of a processing solution to the curled up area would be delayed to cause so-called "uneven development". In addition, the curled-up film would be crushed in the machine and the film being processed would then be "bent or broken".